The adequate management of pain constitutes an important challenge, since currently available treatments provide in many cases only modest improvements, leaving many patients unrelieved [Turk D C, Wilson H D, Cahana A. Treatment of chronic non-cancer pain. Lancet 377, 2226-2235 (2011)]. Pain affects a big portion of the population with an estimated prevalence of around 20% and its incidence, particularly in the case of chronic pain, is increasing due to the population ageing. Additionally, pain is clearly related to comorbidities, such as depression, anxiety and insomnia, which lead to important productivity losses and socio-economical burden [Goldberg D S, McGee S J. Pain as a global public health priority. BMC Public Health. 11, 770 (2011)]. Existing pain therapies include non-steroidal anti-inflammatory drugs (NSAIDs), opioid agonists, calcium channel blockers and antidepressants, but they are much less than optimal regarding their safety ratio. All of them show limited efficacy and a range of secondary effects that preclude their use, especially in chronic settings.
As mentioned before, there are few available therapeutic classes for the treatment of pain, and opioids are among the most effective, especially when addressing severe pain states. They act through three different types of opioid receptors (mu, kappa and gamma) which are transmembrane G-protein coupled receptors (GPCRs). Still, the main analgesic action is attributed to the activation of the μ-opioid receptor (MOR). However, the general administration of MOR agonists is limited due to their important side effects, such as constipation, respiratory depression, tolerance, emesis and physical dependence [Meldrum, M. L. (Ed.). Opioids and Pain Relief: A Historical Perspective. Progress in Pain Research and Management, Vol 25. IASP Press, Seattle, 2003]. Additionally, MOR agonists are not optimal for the treatment of chronic pain as indicated by the diminished effectiveness of morphine against chronic pain conditions. This is especially proven for the chronic pain conditions of neuropathic or inflammatory origin, in comparison to its high potency against acute pain. The finding that chronic pain can lead to MOR down-regulation may offer a molecular basis for the relative lack of efficacy of morphine in long-term treatment settings [Dickenson, A. H., Suzuki, R. Opioids in neuropathic pain: Clues from animal studies. Eur J Pain 9, 113-6 (2005)]. Moreover, prolonged treatment with morphine may result in tolerance to its analgesic effects, most likely due to treatment-induced MOR down-regulation, internalization and other regulatory mechanisms. As a consequence, long-term treatment can result in substantial increases in dosing in order to maintain a clinically satisfactory pain relief, but the narrow therapeutic window of MOR agonists finally results in unacceptable side effects and poor patient compliance.
The sigma-1 (σ1) receptor was discovered 35 years ago and initially assigned to a new subtype of the opioid family, but later on and based on the studies of the enantiomers of SKF-10,047, its independent nature was established. The first link of the σ1 receptor to analgesia was established by Chien and Pasternak [Chien C C, Pasternak G W. Sigma antagonists potentiate opioid analgesia in rats. Neurosci. Lett. 190, 137-9 (1995)], who described it as an endogenous anti-opioid system, based on the finding that σ1 receptor agonists counteracted opioid receptor mediated analgesia, while σ1 receptor antagonists, such as haloperidol, potentiated it.
Many additional preclinical evidences have indicated a clear role of the σ1 receptor in the treatment of pain [Zamanillo D, Romero L, Merlos M, Vela J M. Sigma 1 receptor: A new therapeutic target for pain. Eur. J. Pharmacol, 716, 78-93 (2013)]. The development of the σ1 receptor knockout mice, which show no obvious phenotype and perceive normally sensory stimuli, was a key milestone in this endeavour. In physiological conditions the responses of the σ1 receptor knockout mice to mechanical and thermal stimuli were found to be undistinguishable from WT ones but they were shown to possess a much higher resistance to develop pain behaviours than WT mice when hypersensitivity entered into play. Hence, in the σ1 receptor knockout mice capsaicin did not induce mechanical hypersensitivity, both phases of formalin-induced pain were reduced, and cold and mechanical hypersensitivity were strongly attenuated after partial sciatic nerve ligation or after treatment with paclitaxel, which are models of neuropathic pain. Many of these actions were confirmed by the use of σ1 receptor antagonists and led to the advancement of one compound, S1RA, into clinical trials for the treatment of different pain states. Compound S1RA exerted a substantial reduction of neuropathic pain and anhedonic state following nerve injury (i.e., neuropathic pain conditions) and, as demonstrated in an operant self-administration model, the nerve-injured mice, but not sham-operated mice, acquired the operant responding to obtain it (presumably to get pain relief), indicating that σ1 receptor antagonism relieves neuropathic pain and also address some of the comorbidities (i.e., anhedonia, a core symptom in depression) related to pain states.
Pain is multimodal in nature, since in nearly all pain states several mediators, signaling pathways and molecular mechanisms are implicated. Consequently, monomodal therapies fail to provide complete pain relief. Currently, combining existing therapies is a common clinical practice and many efforts are directed to assess the best combination of available drugs in clinical studies [Mao J, Gold M S, Backonja M. Combination drug therapy for chronic pain: a call for more clinical studies. J. Pain 12, 157-166 (2011)]. Hence, there is an urgent need for innovative therapeutics to address this unmet medical need.
As mentioned previously, opioids are among the most potent analgesics but they are also responsible for various adverse effects which seriously limit their use.
Accordingly, there is still a need to find compounds that have an alternative or improved pharmacological activity in the treatment of pain, being both effective and showing the desired selectivity, and having good “drugability” properties, i.e. good pharmaceutical properties related to administration, distribution, metabolism and excretion.
Thus, the technical problem can therefore be formulated as finding compounds that have an alternative or improved pharmacological activity in the treatment of pain.
In view of the existing results of the currently available therapies and clinical practices, the present invention offers a solution by combining in a single compound binding to two different receptors relevant for the treatment of pain. This was mainly achieved by providing the compounds according to the invention that bind both to the μ-opioid receptor and to the σ1 receptor.